1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
When such fixing device is installed in a color image forming apparatus for forming a color toner image on a recording medium, the fixing device is required to apply heat to the color toner image in an amount sufficient to apply gloss to the toner image on the recording medium with shortened warm-up time. To address this requirement, the fixing device may include a belt that contacts the recording medium bearing the toner image to apply heat to the recording medium. The belt may have a smaller thermal capacity compared to a roller, and therefore may be heated faster than the roller. Moreover, the belt may be laid over a plurality of rollers to lengthen a fixing nip defined by and between the belt pressing against a parallel pressing member and through which the recording medium bearing the toner image passes to enable the fixing device to fix the toner image on the recording medium.
FIGS. 1A and 1B illustrate a fixing device 11R including such belt configuration. A belt 22R is laid over a first roller 20R and a second roller 21R. A pressing member 25R disposed opposite the first roller 20R and the second roller 21R presses against the first roller 20R and the second roller 21R via the belt 22R. The first roller 20R and the second roller 21R disposed adjacent to the first roller 20R may form a substantially flat nip N of great length represented by a combination of a nip section N1 formed between the first roller 20R and the pressing member 25R via the belt 22R, a nip section N2 formed between the belt 22R and the pressing member 25R, and a nip section N3 formed between the second roller 21R and the pressing member 25R via the belt 22R. The great length of the nip N may allow the belt 22R to contact the recording medium for a longer time to apply a greater amount of heat to the recording medium passing through the nip N, which is desirable for good fixation of the toner image on the recording medium. In addition, the flatness of the nip N may prevent any difference between the linear velocity at the nip N of the belt 22R facing the front side of the recording medium, which bears the toner image, and the linear velocity of the pressing member 25R facing the back side of the recording medium, which does not bear the toner image, thus suppressing image shift of the toner image on the recording medium.
However, at the intermediate nip section N2, the pressing member 25R is not pressed against the first roller 20R and the second roller 21R. Accordingly, the pressing member 25R presses the recording medium against the belt 22R with reduced pressure at the nip section N2 compared to the nip sections N1 and N3, resulting in variation in pressure applied at the nip sections N1, N2, and N3.
To address this problem, the fixing device 11R may be configured so that the pressing member 25R applies greater pressure on the belt 22R. However, the greater pressure may degrade durability of both the pressing member 25R and the belt 22R. Alternatively, the first roller 20R and the second roller 21R may be given greater radii, that is, made larger. However, generally the larger the first roller 20R and the second roller 21R the greater their thermal capacity, resulting in increased warm-up time of the upsized fixing device.